How to Grow Cannabis in Soil

Growing in soil is the most natural cultivation method we have. Through understanding the properties of soil and how it takes care of a cannabis plant, growers now have the information to cultivate even better crops. It is after all the foundation on which we have developed all of our knowledge about growing marijuana. 

With the introduction of indoor growing and hydroponics, more growers have stopped using soil but we're here to show you the benefits of keeping it organic with a good old bit of dirt in a pot. This article teaches you everything you need to know about how to grow cannabis in soil.

Growing in Soil: The Pros and Cons

If you are wondering whether to use soil for your next grow, it may help to know the pros and cons of doing so. Growing in soil is a very reliable method of cultivating cannabis and has many benefits. It is full of beneficial bacteria and fungi that aid root function, as well as being extremely affordable and easy to use.

On the other hand, soil can be a slight inconvenience for a number of reasons including reduced water efficiency, heavier lifting, and an increased chance of pests or other harmful bacteria. Every technique has its drawbacks, so take a minute to understand what difficulties you might face if you choose soil as your grow medium.

Pros of Growing in Soil

• Affordable - Growing cannabis doesn't have to be expensive. Compared to hydroponics and other soilless methods of growing, soil is definitely the cheapest option to set up. Not to mention that soil is already a freely available medium in nature. Depending on your region, the earth in your back garden could just be the perfect blend for cultivating lots of healthy cannabis plants.
• Full of Beneficial Bacteria and Fungi - There is a symbiotic relationship between roots and the soil housing them. Beneficial bacteria such as rhizobacteria help prevent disease, and aid nutrient absorption, while beneficial fungi help roots maintain hydration, as well as decomposing organic residue into more food for plants. 
• Sustainable/Eco Friendly - The nutrients used in soil are often organic and better for the environment in general. Outside you can even create your own super soils with food waste and composting. 
• Product Can Taste Better - Cannabis grown in soil is far more natural than hydroponics. Even though hydroponics can produce denser, cannabinoid rich buds, it does not have the same organic, environmental benefits soil grown weed does. Microorganisms naturally present in soil can allow for broader development of terpenes and other substances. 
• Easy to Use - Requires less maintenance than other cultivation practices. Providing a high quality soil mix means you can worry less about pH fluctuations or nutrient imbalances. Setting it up is easier than a hydroponics system.

Cons of Growing in Soil

• Heavy/Inconvenient - If you've ever grown with soil you will know that moving sacs of it to and from your house can be a real task. It also takes longer to fill and clean pots, potentially leaving a lot of mess in the process.
• Attracts Pests - Soil can be the perfect home for beneficial organisms, but that also means it can invite unwanted bugs into your garden. There isn't really a way to completely prevent this other than keeping the environment as clean as possible. There are organic supplements on the market that can be added to soil to help strengthen plants and improve defences against pests.
• Reduced Water Availability - Good quality soil drains water efficiently. Although this is a positive thing, it means plants do not end up using a percentage of the water given to them because it drains out or evaporates. Roots also have to work harder to find/absorb water and nutrients.
• Slower Growth - Typically cannabis grows slower in soil because more energy is spent on root development and nutrient delivery during the vegetative phase.
• Less Control Over Feeding - Mineral build up or imbalance in the soil can make it hard to keep track of what nutrients are available to plants at any given stage.

Different Types of Soil

Nowadays there are so many types of commercial soils available to buy that it can be confusing to know which one will suit your Mary Jane best. Not all soil is the same, so choosing the correct mixture will determine how successful (or unsuccessful) your growing efforts will be.

There are 4 main types of soil commonly used in the cannabis industry. Usually, at least 2 of these different soils are combined, but it varies from brand to brand. You want to make sure it is as fresh as possible and contains adequate levels of good quality microorganisms and fungi. 

Clay Soil

Clay soil is made up primarily of tiny crystalline minerals. It provides an excellent amount of nutrients to plants in a near readily available form. Clay soil absorbs and holds water efficiently but the downside to this is poor drainage due to it being so compact, making it heavy and difficult to use unless mixed with other grow mediums.

• Pros -  Good for organic growing, holds water, nutrient rich
• Cons - Compact, poor drainage, high pH

Sandy Soil

Sandy Soil is often found in a soil mix for being light, airy and offering good drainage. Particles in sandy soil are fairly large and coarse, meaning they help to circulate oxygen but do not retain water effectively. Nutrients can get washed out easily, requiring you to water more regularly.

• Pros -  Efficient drainage, oxygenated soil, easy to use
• Cons - Does not retain water, minerals wash out easily, dries quickly, low pH

Silt Soil

Commonly known as mud, the texture of silt soil is somewhere between that of clay and sand. It is made up of medium sized, floury particles that clump together and store water when irrigated. Silt soil is fertile and full of beneficial organic substances and nutrients needed for healthy plant growth.

• Pros - Rich in organic nutrients, fertile, retains water
• Cons - Medium drainage efficiency, becomes hard/crusty once dry

Loam Soil

Loam Soil is the highest quality soil out of the ones we just mentioned. In fact, loam soil contains all 3 of them. This mixture of clay, sand and silt particles results in an extremely fertile soil with plenty of benefits including good filtration of both water and oxygen.

• Pros - optimal drainage, well balanced water retention, effective aeration
• Cons - Expensive, can compact, dries faster than clay

The Right Soil Mix: Amendments

Cannabis plants need plenty of aeration and a medium with good drainage in order to thrive. Some soils will not provide all the necessary components to sustain plants throughout their whole cycle and nutrients will need to be added at different stages.

This is the benefit of creating your own super soil, which already contains most of the required nutrition. However, making a super soils can be time consuming so most growers go for store bought options and add amendments to improve effectiveness.

Coco Coir - Coco coir is made from coconut husks and works incredibly efficiently for its ability to retain water. It has some levels of nutrients such as potassium, which is consumed by a cannabis plant in larger quantities during flowering.

Vermiculite or Perlite - These are great additions if you are looking to improve aeration. Either or both can be used in your soil mix to aid air and water dispersion due to their light, airy structure.

Peat Moss - Moss can hold many times its weight in water. Strengthens the roots through the development of antibacterial organisms that help to break down organic matter into consumable nutrients. Good for lowering the pH of soil.

Worm Castings - Castings are basically worm faeces, and cannabis plants love them, especially during the vegetative cycle. They provides plants with plenty of nitrogen and contain other beneficial bacteria. 

Blood Meal - Another nitrogen rich fertilizer that works particularly well during vegging. This organic amendment is made from animal blood, meaning it also has adequate levels of iron in it. 

There are many amendments you can look into such as compost, kelp meal, bone meal, mycorrhizal fungi, Epsom salts, all of which have their own uses to boost your soil.

Pots for Growing in Soil

Choosing the right pot for your soil grow is critical if you are looking to maximise efficiency and yields. Using a huge container for a small plant isn't the best idea because it can be hard to manage the water content at any given time. Pots need to be sized in relation to the root mass at different stages of the plant's vegetative stage. We recommend using one gallon per foot of plant height. 

Most growers will transplat 1 - 2 times into larger pots during the vegetative phase. Starting in small pots allows roots to make best use of the available water in the soil while they are still young. As the soil dries, roots are encouraged to grow in search of water. Once the roots have filled a pot, the plant can be transplanted into a bigger container.

Roots can be checked by placing your hand palm down on the top soil with the main stem between your middle finger and ring finger and turning the plant upside down. Lift the pot off by gently wiggling it. If the roots are becoming heavily pot bound you can consider transplanting.

Pots should be big enough so that you are watering every 1 - 3 days. Cannabis roots need plenty of oxygen so it is important that the grow medium doesn't stay wet for long periods of time to avoid problems. Cannabis plants that do not get a proper dry/wet cycle can suffer from problems such as stunted growth, drought, oxygen depravation, mold or rot.

Indoor soil grow by Wascanna from GrowDiaries

Soil Maintenance

Watering

One of the biggest problems with soil growing is the risk of overwatering. Whether you are using soil or hydroponics, it is important that roots get regular watering to avoid roots drying out. Too much, and plants will not be able to function correctly.

As we mentioned before, cannabis enjoys a dry/wet cycle so get the feel for how much your plants are drinking and water accordingly.

There are 2 ways we check to see if soil is dry enough for watering:

1. Use your finger to feel the dryness of the top soil. Stick your finger vertically down into the soil. If you do not feel much moisture then plants probably need watering. 
2. Use both hands to feel the weight of the pot. Try checking the weight of the pot before and after watering so you get used to the difference. 

pH levels

The pH level of your soil should be kept within a certain range. If pH levels turn too acidic or too alkaline a cannabis plant will not be able to absorb nutrients correctly, causing it some real issues. The ideal soil pH range for growing cannabis is between 6 and 7.

Soils maintain their pH fairly well, however it is worth using a pH meter to keep track of any fluctuations. Digital meters or drop kits can be used for measuring water and nutrient solutions. Each pot can also be measured individually with a soil meter as plants may be consuming nutrients at different rates.

When nutrients are added to water the pH changes. Through feeding, minerals and salts eventually build up in the soil, which increases its acidity. This can be regulated by using a pH up or down supplement in your solutions before watering. 

We recommend you take a reading of the soil's pH at least every week or two to make sure the pH maintains within the desired range. Bear in mind that cannabis benefits from slight changes in pH in order to properly uptake the full spectrum of necessary minerals.

Nutrients

If you are working with a super soil, then it may not be necessary for you to add extra nutrients but they can still come in handy if your plants need help somewhere or become deficient.

The three most important nutrients your plants should be getting are nitrogen, phosphorous and potassium (NPK). There are also some essential trace minerals (often found in soil nutrients) present in soil which need to be considered including iron, copper, magnesium, calcium, boron, manganese and zinc. 

Cannabis plants will use up some nutrients faster than others at different stages of growth. We can enrich a store bought soil by adding certain soil nutrients either before or after planting.

Marijuana grows well with roughly the following NPK levels. Remember that different strains can handle higher or lower doses of nutrients. Check the mineral concentration of your fertilizer and soil before adding.

Example of a cannabis plant that did not receive enough nitrogen:

Flushing

Flushing your soil at the end of a grow will help remove any excess build up of nutrients in the growing medium, leaving you with much tastier bud. Flushing in soil should start no later than 2 weeks before harvest. Flushing for as long is less important if you have grown organically and only used water for the majority of the grow cycle.

You can also give plants pure water between feedings to avoid the concentration of these salts becoming too high.

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Growing Cannabis in Soil: Tips

Indoor

The size of your cannabis plants will be determined by a few factors, including pots, humidity, light, temperature, oxygen and available nutrients. Growing indoors means you likely have limited space so consider realistically how big you plan to grow your plants and then decide how many you can fit into the available space. This will also help you figure out what pot size could work best.

It is best to leave enough space to move around and work with your cannabis plants. Working with soil can become a proper mess if you are dealing with it indoors.

Tips for growing indoors:

• Use root boosters (rhizobacteria) in the early stages of growth until flowering.
• Place drip trays underneath your pots so that any excess water is collected at the base of the container.
• Keep your grow space as clean as possible by clearing up dead leaves, dirt and dust regularly.
• Cannabis plants need sufficient space in order to benefit from their containers.

Outdoor

It is common to start cannabis plants in soil indoors in a small pot, to be later transplanted outside. Outdoor cannabis plants are usually either grown in pots or planted directly into the ground.

If you are planting straight into the soil, you will probably need to treat it beforehand. Soil is an abundant resource outside but not all of it will be good enough quality for cannabis. After checking its pH you can make adjustments with the various soil amendments and supplements.

Tips for growing outdoors:

• Test the pH before planting in any soil to see if it is suitable for cannabis.
• Start a compost pile so you have a nutrient rich medium to use in your next grows.
• Add beneficial fungi such as mycorrhizal to the soil mix to protect roots from invasive pests.
• Choose genetics designed for soil and the outdoors.

Conclusion

Growing in soil is very forgiving and is a great place to start if you are a beginner either looking to grow indoors or outdoors. It is a great way to learn the basics of cannabis cultivation and will have you growing healthy plants with minimal effort and investment. You will soon see how easy it can be to get it all up and running.

Experiment with different soil mixes and strains to see how the composition affects growth and yield. Once you find a reliable soil blend that works well with your plants you can fine tune it so your garden has exactly what it needs to produce big, sticky buds.

External References

Plant Growth-Promoting Rhizobacteria for Cannabis Production: Yield, Cannabinoid Profile and Disease Resistance. Frontiers in Microbiology. - Lyu, Dongmei & Backer, Rachel & Robinson, W. & Smith, Donald. (2019)

Nitrogen Mineralization and Nitrification in Two Soils with Different pH Levels. Communications in Soil Science and Plant Analysis. - Bergamasco, Marina & Braos, Lucas & Lopes, Ivã & Cruz, Mara. (2019)

Characterization of Nutrient Disorders of Cannabis sativa. Applied Sciences. - Cockson, Paul & Landis, Hunter & Smith, Turner & Hicks, Kristin & Whipker, Brian. (2019)

This article was updated September 2020